Chemical process data 
#from "The Estimation of Sigma for an X Chart: MR/d2 or
#S/c4", Jonathan D. Cryer and Thompas P. Ryan, manuscript, February, 1989
.67
.63
.76
.66
.69
.71
.72
.71
.72
.72
.83
.87
.76
.79
.74
.81
.76
.77
.68
.68
.74
.68
.69
.75
.80
.81
.86
.86
.79
.78
.77
.77
.80
.76
.67

